1. What frequency clock source will produce clock waveforms having a period equal to 5 us (5 microseconds)?

Note:    M = 10^6 
         k = 10^3 
         m = 10^-3 
         u = 10^-6

1 is CORRECT. Using the formula

f = 1/T, where T = 5 us or 5 x 10^-6 s,

results in the following order of value * prefix * unit

f = 1/5 * 1/u * 1/s 
  = .2  * 10^6 * Hz 
  = .2  * M   * Hz
  = .2 MHz

Also note that .2 MHz is also equal to 200 kHz!